Make -Wformat accept printf("%hhx", c); with -funsigned-char

For "%hhx", printf expects an unsigned char. This makes Clang
accept a 'char' argument for that also when using -funsigned-char.

This fixes PR12761.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156388 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/FormatString.cpp b/lib/Analysis/FormatString.cpp
index ab69c06..f776c89 100644
--- a/lib/Analysis/FormatString.cpp
+++ b/lib/Analysis/FormatString.cpp
@@ -265,10 +265,9 @@
             break;
           case BuiltinType::Char_S:
           case BuiltinType::SChar:
-            return T == C.UnsignedCharTy;
           case BuiltinType::Char_U:
           case BuiltinType::UChar:                    
-            return T == C.SignedCharTy;
+            return T == C.UnsignedCharTy || T == C.SignedCharTy;
           case BuiltinType::Short:
             return T == C.UnsignedShortTy;
           case BuiltinType::UShort: